1. Field of the Invention
The present invention relates to an optical disc tilt control apparatus, and more particularly, to an apparatus for controlling the inclination of the optic axis of an optical pickup with respect to the information surface of an optical disc.
2. Description of the Related Art
In some prior art optical disc apparatuses, reproduction of a signal recorded on the optical disc is performed by causing to converge onto an optical disc that is rotating with a prescribed rotational speed a light beam that is generated from a light source such as a semiconductor laser. An example of an optical disc will be described with reference to FIG. 35. A plurality of tracks are formed on the disc in spiral fashion. FIG. 35 is a diagram showing the cross-section of a disc. Tracks are formed by grooves and lands. Each groove or each land constitutes a track. The track pitch is 0.74 micrometers (hereinbelow abbreviated as xcexcm). A recording film consisting of phase changing material etc. is affixed to the recording surface. When information is recorded on the disc, the coefficient of reflectivity of the recording film changes due to changes in the intensity of the light beam that are produced in accordance with the information whilst tracking control is exercised such that the light beam is always positioned on the track. When the information on the track is reproduced, the reflected beam from the optical disc is photodetected by a photodetector whilst exercising tracking control such that the light beam is positioned on the track. The information is reproduced by processing the output of the optical detector.
The addresses will be described with reference to FIG. 36.
The portions indicated by a sequence of xe2x80x9cpitsxe2x80x9d constitute the header field.
The xe2x80x9cpitsxe2x80x9d are of land shape. The header field is arranged at the head of each sector. The sequence of pits is arranged at a position between a land track and a groove track. This arrangement of the header field is generally called CAPA (Complementary Allocated Pit Addressing). The header field is constituted by a variable frequency oscillator (hereinbelow abbreviated to VFO) and sector address. VFO1 and VFO2 are recorded at a single frequency and are employed to pull in the phase locked loop (hereinbelow called PLL). Sector address 1 indicates the address of the groove sector and sector address 2 indicates the address of the land sector.
The disc is divided in the radial direction into several zones. The number of sectors per track in each zone is fixed. Going from the inner circumferential to the outer circumferential zone, the number of sectors per track increases.
When performing information recording, this is performed after exercising control such that the speed of rotation of the disc is a speed of rotation corresponding to the respective zone. The linear speed in each zone is therefore practically constant. The region apart from the header field is a region in which information re-writing can be performed. Hereinbelow it is termed the xe2x80x9cre-writable regionxe2x80x9d.
Detection of the offset of a track and the light beam for tracking control is likewise obtained from the reflected light from the disc. A tracking error detection system generally called the xe2x80x9cpush-pullxe2x80x9d system will now be described.
Hereinbelow, tracking error is abbreviated as TE. The push-pull method is also called the xe2x80x9cfar fieldxe2x80x9d method. This is a system in which the TE signal is detected by using a photodetector divided into two photodetection sections arranged symmetrically with respect to the track center to extract as output difference the light reflected and diffracted by guide grooves on the disc. As shown in FIG. 37, a left-right symmetrical reflection/diffraction distribution is obtained when the spot of the light beam coincides with the center of the land or the center of the groove. Otherwise, the optical intensity is different for left and right. FIG. 38 shows the output difference of the outputs from two sections of the photodetector divided into two when the spot crosses a track. The TE signal becomes zero at the center of the land track or the center of the groove track. Tracking control is performed by moving the spot on the disc in the direction orthogonal to the track in accordance with the TE signal. Movement of the spot of the light beam in the direction at right angles to the track is achieved by moving a converging lens by means of a tracking actuator.
The TE signal characteristic shown by the solid line in FIG. 38 shows the case where the optic axis of the light beam is perpendicular to the information surface of the disc. The case where the optic axis of the light beam is inclined in the radial direction of the disc is shown by the dotted lines. Hereinbelow, the inclination of the radial direction of the optic axis of the light beam from a surface perpendicular to the information surface of the disc will be called the xe2x80x9cradial tiltxe2x80x9d.
The phase of the TE signal is offset by the radial tilt. Specifically, the spot is offset from the center of the track even if tracking control is exercised such that the TE signal becomes zero. With NA=0.6, wavelength 650 nm, track pitch 0.6 xcexcm, groove depth of wavelength/6, and 50% duty of lands and grooves, an inclination of 1xc2x0 generates a tracking error of about 0.13 xcexcm. Some differences are produced by differences in the Gaussian optical intensity distribution of the light beam. In some optical disc apparatuses, a radial tilt of about 1xc2x0 may be produced by inclination of the disc or inclination of the turntable etc. of the disc motor.
As described above, if a radial tilt of about 1xc2x0 is produced by disc inclination or inclination of the turntable etc. of the disc motor etc., this generates a tracking error of 0.13 xcexcm, so reproduction of information becomes unstable and during recording the information of adjacent tracks may be erased.
An object of the present invention is to provide an optical disc apparatus wherein stable reproduction of information can be achieved even when radial tilting has occurred and wherein there is no possibility of the information of adjacent tracks being erased when recording.
An optical disc apparatus according to the present invention for achieving the above object comprises: means for reproduction signal detection that detect information recorded on a disc by convergently directing a light beam onto a disc on which there are arranged a first pit sequence formed in a position offset in one direction orthogonal to a track and a second pit sequence formed in a position offset in the other direction orthogonal to the track; first means for detecting tracking error that detect positional offset of the track and the light beam by a push-pull method; second means for detecting tracking error that detect positional offset of the track and the light beam from the reproduction signals of the first pit sequence and of the second pit sequence output by the means for reproduction signal detection; means for effecting movement that effect movement such that the light beam moves transversely across the track; means for tracking control that control the means for effecting movement in accordance with the output of the first means for detecting tracking error such that the light beam is positioned on the track; and means for correction by altering the target position of the means for tracking control in accordance with the output of the second means for detecting tracking error.
There are also provided: means for reproduction signal detection that detect information recorded on a disc by convergently directing a light beam onto a disc on which there are arranged a first pit sequence formed in a position offset in one direction orthogonal to a track and a second pit sequence formed in a position offset in the other direction orthogonal to the track; first means for detecting tracking error that detect positional offset of the track and the light beam by a push-pull method; second means for detecting tracking error that detect positional offset of the track and the light beam from the reproduction signals of the first pit sequence and of the second pit sequence output by the means for reproduction signal detection; means for effecting movement that effect movement such that the light beam moves transversely across the track; means for angle variation that change the angle of incidence of the light beam that is directed onto the information surface; means for tracking control that control the means for effecting movement in accordance with the output of the first means for detecting tracking error such that the light beam is positioned on the track; means for correction by altering the target position of the means for tracking control in accordance with the output of the second means for detecting tracking error; and means for angle control that control the means for angle variation in accordance with the output of the first means for detecting tracking error when the means for correction are actuated.
With an optical disc apparatus according to the present invention, with the above construction, the target position of tracking control using a TE signal obtained by a push-pull method is corrected using a second TE signal that detects positional offset of the light beam and the track using the reproduction signal when the light beam passes over the first pit sequence formed in a position offset in one direction orthogonal to the track and a second pit sequence formed in a position offset in the other direction orthogonal to the track, so tracking offset due to radial tilt can be corrected and the spot controlled to the center of the track.
Also, since radial tilt is controlled using a TE signal obtained by a push-pull method when the target position of tracking control using a TE signal obtained by a push-pull method is corrected using a second TE signal that detects positional offset of the light beam and the track using the reproduction signal when the light beam passes over the first pit sequence formed in a position offset in one direction orthogonal to the track and the second pit sequence formed in a position offset in the other direction orthogonal to the track, the optic axis of the light beam becomes perpendicular to the information surface of the disc.